Influence of Environment 231 



B 



FIG. 84. THE THREE CASTES OF THE HONEY BEE. A, worker or imper- 

 fect female ; B, queen or perfect female ; C, drone or male. The differ- 

 ences betwen workers and queens are produced by the type of food sup- 

 plied to the larvae. 



or in instincts are determined by the character of the food and 

 not by heredity. Innumerable cases of a similar sort could be 

 named which show the great effect of environmental stimuli on 

 development but not upon heredity. 



C. FUNCTIONAL ACTIVITY AS A FACTOR OF DEVELOPMENT 

 Another factor of development which is partly intrinsic and part- 

 ly extrinsic is functional activity or use. Functional activity is re- 

 sponse to stimuli which may be external or internal in origin. The 

 entire process of development may be regarded as a series of such 

 responses on the part of the organism, whether germ cell, embryo 

 or adult. The nature of the response is determined by the nature 

 and state of the organism and by the character of the stimulus. 

 By the normal, or usual, series of stimuli certain parts are kept 

 active while other parts are kept inactive or are inhibited. 



Developmental Movements. Normal development is depend- 

 ent on the correlated activity of many parts of the organism. If 

 in any part stimuli and responses are lacking the development 

 of that part is arrested or inhibited. For example in the cleav- 

 age stages different substances are sorted and localized by pro- 



